The present invention relates to a method of extending the secondary creep life of alloys, such as those used for turbine blades and for other high-temperature high-stress applications. The invention is particularly useful with respect to nickel-base superalloys but can also advantageously be used for increasing the creep life of other alloys, such as those of cobalt, iron, titanium, and aluminum.
The term "creep" refers to the deformation or dimensional changes experienced by a metal part when subjected to sustained stress at elevated temperatures. Such dimensional changes are generally recognized as being divided into three stages, namely: (1) the initial stage, called primary creep, wherein the rate of deformation is relatively large at first, and then gradually decreases; (2) the secondary stage, called secondary creep, wherein the rate of deformation is a relatively steady one; and (3) the final stage, called tertiary creep, wherein the rate of deformation at first gradually increases and then more sharply increases until rupture occurs.
A large number of superalloys have been developed providing high tensile, creep and fatigue strength at elevated temperatures. Such alloys are usually first subjected to a solution heat treatment at a high temperature in order to dissolve the alloy ingredients, and then to a precipitation hardening heat treatment at a lower temperature to precipitate them from the solid solution. Creep-resistant properties are extremely important in aircraft engines, for example, particularly for making turbine blades and discs which are subject to high, sustained tensile stresses at elevated temperatures by the centrifugal forces acting upon them in service. In order to realise maximum efficiency, gas turbine engines are operated under conditions which, in many cases, cause the creep lives of the turbine blades and discs to be limited. During periodic overhauls, the dimensions of the blades are usually measured, and when they exceed a predetermined tolerance, due to creep, the blades are scrapped. Such blades are very costly to produce, and therefore their scrapping involves a very substantial expense. Furthermore, there are cases where complete mechanical failure of a blade has occurred without its having reached its expected permissible life. In certain cases, discs, which can cost tens of thousands of dollars, are scrapped at fixed time periods of service even without their showing signs of damage, simply because of the fear that they may fail catastrophically on further service because of creep.
For the foregoing reasons, considerable efforts have been devoted to providing a method of increasing the safe working life of turbine blades and discs. Some of these efforts have been directed towards attempts to cure creep damage by various heat treatments mainly in the tertiary stage of creep, but this work has met with very limited success. Other work has been directed towards applying a "regenerative" heat treatment to the superalloy part, in which the original solution heat treatment at a high temperature and subsequent precipitation hardening heat treatment at a lower temperature, were repeated after the part had sustained creep during service, but this work has also met with very limited or no success.